Pressure swing adsorption (PSA) processes provide an efficient and economical process for separating a multicomponent gas stream that contains at least two gases that have different adsorption characteristics. One of the gases may be preferentially adsorbed and can be an impurity that is separated from the other gas, which may be taken off as product. Alternatively, the gas that is preferentially adsorbed can be the desired product, which is separated from the other gas. For example, it may be desired to remove carbon dioxide, carbon monoxide, and light hydrocarbons from a hydrogen-containing feed stream to produce an impurity-depleted hydrogen (>99.9 mol % H2) stream for a hydrocracking or hydrotreating process where the impurities, especially carbon monoxide, could adversely affect the catalyst or the reaction.
In pressure swing adsorption processes, the multicomponent gas stream is typically fed to one or more adsorption beds at an elevated pressure to promote adsorption of at least one component, while at least one other component (for example, hydrogen) passes through the adsorption bed. At a defined time, feed to the adsorption bed is terminated and the adsorption bed is depressurized in one or more concurrent depressurization steps wherein pressure is reduced to a defined level that permits the separated, less-strongly adsorbed component or components to be withdrawn from the adsorption bed without significant desorption of the preferentially adsorbed components. Then, the adsorption bed is depressurized in a counter-current depressurization step (blowdown step) wherein the pressure in the adsorption bed is further reduced by withdrawing desorbed gas countercurrent to the direction of the multicomponent feed stream. Following the blowdown step, the bed is purged in a counter-current direction at a low pressure with a purified hydrogen stream to further desorb impurities, thereby creating a low-pressure tail gas stream. In multi-bed adsorption units, there are typically additional steps, and those noted above may be done in stages.
It is particularly desirable to minimize the amount of carbon dioxide and carbon monoxide in the impurity-depleted hydrogen streams. Conventionally, an activated carbon layer is used to remove carbon dioxide. However, still undesirable quantities of CO2 migrate through the activated carbon layer. The migrated CO2 interferes with the removal of other undesirable impurities such as CO and N2 in the subsequent layers. Further, U.S. Pat. No. 4,775,396 teach using zinc or rare-earth exchanged faujasite molecular sieve for removing CO2. However, this would entail significant cost.
Accordingly, it is desirable to provide methods and apparatuses for improving the removal of carbon dioxide in steam reforming based hydrogen PSA units and achieve net gain in performance. Further, it is desirable to provide a cost-effective method and apparatus that protects the sections subsequent to CO2 removal section in a PSA unit from CO2 migration and hence improve their efficiency in removing other impurities. Furthermore, other desirable features and characteristics of the present subject matter will become apparent from the subsequent detailed description of the subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the subject matter.